In many contexts it is desirable to provide small structures in the surface of a material. A known method for removing material in small dimensions is etching. A common field using etching is the production of electric conductors on printed circuits by removing portions of an electrically conductive layer.
In e.g. the electronics industry there is a need of producing smaller and smaller components, for instance by removing material in very small dimensions by etching. Today it is possible to produce etched structures having a width and depth below 1 .mu.m.
For producing such small structures by etching, it is also desirable to remove material to varying degrees in different directions, i.e. to control the etching effect of an etchant in different directions. Etching with the same etching effect in all directions is generally called isotropic etching, whereas etching with a varying etching effect is called anisotropic etching.
When making small structures by etching, use is today made of different etching methods. These can be divided into dry methods and wet methods. Dry etching methods include, for instance, ion-beam etching which is a mechanically machining method, and plasma etching which is a chemically machining method. Wet etching methods include chemical etching and electrochemical etching.
In mechanical methods, such as ion-beam etching, a surface which is to be etched is bombarded with high-energy ions. The ions remove atoms from the surface mechanically. Such etching thus is anisotropic.
In chemically acting dry etching methods, such as plasma etching, the ions are guided to a surface of a substance to be etched by means of an electric field. Such etching is carried out mainly by chemical reactions and therefore is not as anisotropic as purely mechanical etching methods. Some degree of mechanical etching can also take place in chemically acting etching.
In plasma etching, an electric field is applied over a gas. The field is strong enough to make the gas be converted and ionised to form a reactive plasma. Reactive ions are passed by the electric field to a surface to be etched and react therewith in an etching manner.
Dry etching methods are today used in the electronics industry for production of electronic components. Anisotropic etching of structures in small dimensions, 1 .mu.m and less, can be effected.
A serious drawback of the dry etching methods used today is that they are difficult to control since a large number of variables which affect the etching must be kept within strict tolerances. Thus, the technical equipment will be complicated and expensive. The cost of the equipment will also be affected by the size of the workpiece that is to be etched and increases significantly if the equipment is to be dimensioned for the handling of large workpieces.
When producing electronic components, such as integrated circuits and semiconductor components, great requirements are placed on the purity of the components. This requires in dry etching methods, especially in mechanically acting methods, careful cleaning of the etched material, since it has usually been contaminated by residual products. The cleaning operation itself involves an additional step, which besides being time-consuming also requires the use of cleaning agents which in turn have a negative influence on the environment.
To prevent the areas which are not be etched from being affected by the etching, it is common when etching very small structures and necessary in mechanically acting etching methods to mask these areas with a protective layer, called resist. When mechanically affected, for instance by ion bombardment, also the protective layer will be affected during etching. This results in two drawbacks, on the one hand that the protective layer must be very thick so as not to be removed completely in the course of etching and, on the other hand, that the contour of the protective layer towards the surface to be etched becomes uneven owing to the removing effect of the ions, which results in an uneven etched contour.
The substance to be etched is masked also in wet etching methods. Then the substance is immersed in an etching fluid containing an etchant, which, when contacting the substance, is capable of etching.
In chemical etching use is made of an etching fluid containing a solution of an etchant which is capable of etching, by spontaneous chemical reaction, a substance, i.e. the etchant will etch directly when contacting the substance to be etched. The etching occurs isotropically. The etching rate is affected by etching time, temperature and concentration of etchant. The etching fluid usually contains an oxidising agent, for instance BR.sub.2, H.sub.2 O.sub.2, HNO.sub.3, a complexing agent, for instance H.sub.2 SO.sub.4, HF, NaOH, and a solvent, for instance water or methanol. Examples of generally used and recommended compositions of etching solutions for different metals are disclosed in, for instance, "Handbok i metallmikroskopiering" (in English: "Handbook of Metallographic Microscopy"), Helfrid Modin and Sten Modin (1977, Meritforlaget, Johanneshov, Sweden). Typical concentrations of etchant for etching small structures, microstructures, are, for etching of e.g. chromium or copper, 0.8-1.2 M.
Some solvents dissolve a given crystal plane in a substance to be etched more rapidly than other planes, for instance in a semiconductor material, which results in a directionally dependent etching effect, i.e. anisotopic etching.
In electrochemical etching, the etching fluid contains an electrolyte, e.g. a salt solution, which in itself is not capable of etching the substance to be etched by spontaneous chemical reaction, i.e. the etchant does not etch merely by contacting the substance. By applying an electric voltage in the etching fluid between the substance to be etched and an electrode immersed in the etching fluid, an electrolytical process, however, will be begun, in which the substance to be etched is the one pole, usually the anode, and the electrode the other pole. In the electrolytic process, electric current flows in the etching fluid, and ions in the etching fluid react in an etching manner with the substance to be etched.
The etching rate is essentially proportional to the strength of current. The etching will be slightly anisotropic, although not to the same great extent as is possible in dry etching methods. For instance, it is possible to etch in an electrochemical manner structures having a depth-to-width ratio of 1:2.
Several techniques are known for applying voltage in pulses to obtain a good etching effect when using different electrolytes as etching fluid.
In wet etching methods, above all in chemical etching, so-called underetching occurs owing to the isotropic etching properties, i.e. etching off material under the surface that is coated with a protective layer. As a result, it is not possible in purely chemical etching to produce grooves or lines having a greater depth than width. Nor is it possible in electrochemical etching to etch, in case of small dimensions, grooves or lines whose depth exceeds the width. The possibilities of making narrow grooves, e.g. in order to arrange conductors closely together, are thus restricted when using wet etching methods. Furthermore, it is today not possible to produce by wet etching methods even structures, for instance grooves having straight walls, whose width or depth is less than 1 .mu.m.
In wet etching, use is today generally made of fluids which are strongly toxic and harmful to the environment, which in itself is an environmental problem.
Purely chemical etching is also a process which is difficult to control since a plurality of parameters influence the speed of the process.
In electrochemical etching, all surfaces to be etched must be connected to an electric pole during the entire etching procedure. When making printed circuits, this is done by all conductors being interconnected at a connection point during the etching procedure. After completion of the etching, the connection point is removed mechanically in a special production step.
Many experiments have been made, however not quite successful so far, to provide a wet etching method that can be used in the production of small electronic circuits, such as integrated circuits.